Seismic Swarm S20100613.1: Analysis of Activity Near Borrego Springs, California

The region surrounding Borrego Springs lies within the Peninsular Ranges of Southern California, where the interaction between the Pacific and North American plates drives ongoing deformation along multiple fault systems. The San Jacinto Fault Zone, including strands such as the Coyote Creek Fault, dominates local tectonics and accommodates a significant portion of the plate-boundary strain. This setting produces both large-magnitude events and frequent earthquake swarms, reflecting the distributed, interconnected nature of the faults.

Swarm S20100613.1 began at 16:19 on 12 June 2010 and concluded at 22:06 on 21 June 2010, centered 16 km north of Borrego Springs. Over 221 hours and 47 minutes the sequence produced 239 earthquakes. The first 100 events commenced with a magnitude 0.8 shock at 7 km depth. Within hours, two larger events occurred: a magnitude 4.4 earthquake at 11 km depth followed minutes later by a magnitude 4.2 event at 4 km depth. Subsequent activity consisted predominantly of microearthquakes, with magnitudes clustered between 0.1 and 2.6. Depths ranged from 3 km to 17 km, averaging near 8 km, consistent with brittle failure in the upper crust of this fault system.

The temporal pattern showed an initial energetic phase on 13 June that included the two largest events, after which activity declined into a prolonged sequence of smaller shocks. Magnitudes remained modest, rarely exceeding 2.0 after the opening day, while event depths stayed stable within the 4–11 km interval for the majority of the recorded quakes. This distribution aligns with typical swarm behavior in the area, where fluid migration or aseismic slip may trigger successive failures without a clear mainshock-aftershock decay.

Since 1 January 2000 the same locale has hosted ten documented swarms. These occurred in 2001 (one swarm), 2002 (two), 2003 (one), 2005 (two), 2009 (one), and 2010 (three). The 2010 sequence therefore represents the third swarm recorded that year, underscoring elevated swarm frequency during this period.

Such recurrent swarm activity highlights the mechanical complexity of the San Jacinto Fault Zone. Continued monitoring of event rates, depths, and focal mechanisms remains essential for assessing whether future swarms may evolve into larger triggered events along adjacent fault segments.

References

SeismoSight internal swarm catalog (S20100613.1 parameters and event list).
USGS Earthquake Hazards Program regional fault database.
Southern California Seismic Network historical seismicity summaries.